Hyde-Smith responded with a tweet to the president: "I could not be more honored to have your endorsement."

Trump's endorsement has been seen as crucial to Hyde-Smith's campaign. She has faced questions about her Republican and conservative bona fides in Mississippi and Washington. She had served as a Democratic state senator for much of her political career, although she frequently voted with Republicans on key issues. She switched to the Republican Party in 2010 prior to her two successful runs for state agriculture commissioner.

Gov. Phil Bryant, a close ally of Trump's, in April appointed Hyde-Smith to the Senate seat vacated by Republican Thad Cochran. When he announced her appointment, Bryant said he believed a Trump endorsement would be soon forthcoming.

Besides Bryant's close ties to Trump, Hyde-Smith helped the Trump campaign in Mississippi and she had been considered a candidate for Trump's agriculture secretary or another post. She co-chaired the campaign's Agriculture Advisory Committee and had been involved in White House meetings with Trump's Agriculture Secretary Sonny Purdue and Vice President Mike Pence on agriculture issues and tax reform.

But her appointment drew concerns from some state Republican leaders and, reportedly from the White House over whether she could win the race.

Republican challenger state Sen. Chris McDaniel, who also vied for Trump's endorsement, has used Hyde-Smith's Democratic past as his main campaign offensive. In 2014, when he nearly toppled Cochran with a conservative, tea-party fueled campaign, then-businessman Trump endorsed McDaniel.

But McDaniel's run against Hyde-Smith to date hasn't engendered the national conservative support — or funding — his 2014 run did. This also likely played into Trump's decision, as several polls have shown Hyde-Smith putting daylight between herself and McDaniel.

Democratic former U.S. Rep. Mike Espy, a former Clinton cabinet member, is also in the hunt in November, and Republicans have feared a fierce battle between Hyde-Smith and McDaniel would give an opening to turn the decades-long red seat blue.

Trump and Pence had endorsed incumbent Republican Sen. Roger Wicker, but demurred on Hyde-Smith. White House political advisers reportedly met with Hyde-Smith and set benchmarks for her campaign before an endorsement would be considered.

One of those was reported to be fundraising. Hyde-Smith appears to have achieved that goal, having raised nearly $2 million in her first reporting quarter.

In late April, Hyde-Smith's campaign accidentally jumped the gun on a Trump endorsement. A fundraising letter from the campaign said that Trump and Pence support her. The campaign said the statement in the letter was a drafting error in the email generated by an outside vendor hired by the campaign.

Trump had initially urged Bryant to appoint himself to the open Senate seat. Bryant declined, saying such a move would be "unseemly."

Trump's initial reluctance to endorse Hyde-Smith was also likely influenced by his experience with an Alabama Senate race last year. Trump endorsed appointed Sen. Luther Strange in a GOP runoff that Strange lost to Roy Moore. Moore then lost the longtime red seat to Democrat Doug Jones in an historic political upset.

But more recently, Trump's picks have been winning across the country in congressional and gubernatorial GOP primaries. As Trump tweeted Wednesday, he went "5 for 5" in Tuesday's primaries.